A hacker who claims to have stolen thousands of sensitive documents relating to the attack on the Twin Towers is offering to sell them off to terrorist groups for bitcoin.

The FBI is investigating the theft of 18,000 insurance and legal documents, the Financial Times reports.

The hacker, which is posting as “The Dark Overlord,” claimed this week it has stolen emails and nondisclosure agreements about the attacks that were shared between insurers Hiscox and Lloyd’s of London as well as law firm Husch Blackwell, formerly Blackwell Sanders Peper.

The group has invited ISIS, al-Qaeda and others to bid for documents.

A spokesperson for Silverstein Properties, which signed a $3.2B, 99-year ground lease on the towers six weeks before the attack, said it is doing an internal investigation but as yet has found no evidence that a breach has taken place at the company.

“We are aware of claims of alleged security breaches at firms involved in the five-year insurance litigation following the attacks of 9/11, and are conducting an internal investigation based on these claims,” the spokesperson said in an email. “We have spent the last 17 years fulfilling our obligation to deliver a magnificent and fully rebuilt World Trade Center. We will not be distracted by 9/11 conspiracy theories.”

Hiscox has known since April that a system one of its legal advisers uses was hacked, and said that it is working with authorities on the matter. Husch Blackwell told the FT it has not been hacked, and Lloyd’s of London said it has not yet found any evidence its systems have been breached.

“The Dark Overlord” has a history of extorting companies, and threatening to release documents unless its victims pay. The group said that solicitors, politicians, law enforcement agencies, insurers and others should contact the group if they want their documents removed from the cache, Motherboard reports.

“However, you'll be paying us,” the group wrote.